Sulphonic acids of aromatic di- and polyisocyanates are known. They are prepared simply by the reaction of the corresponding aromatic di- or polyisocyanates with sulphonating agents such as sulphur trioxide, adducts of sulphur trioxide, oleum, chlorosulphonic acid or sulphuric acid as described for example in German Published patent application No. 2,227,111 and its British concordance British Pat. No. 1,383,184; German Published patent application No. 2,359,619 and its U.S. concordance U.S. Pat. No. 3,959,329; and U.S. Pat. No. 3,826,769.
The sulphonation products may be obtained as solid, resinous or pulverulent substances or as solutions of the sulphonated isocyanates in unchanged starting isocyanate, depending on the isocyanate used and the degree of sulphonation.
Although there are no difficulties in handling liquid sulphonated polyisocyanates, the preparation, storage and use of solid pulverulent isocyanatosulphonic acids poses considerable problems.
The products are frequently obtained in such a finely divided state that they are difficult to separate and free from the liquid reaction medium. The dried products rise up as dust when poured into containers. Most importantly, dry powders produced by this method are not stable in storage. The melting and decomposition point rise during storage and the products become progressively more insoluble in organic solvents and in the polyesters, polyethers and polyols normally used for the preparation of polyurethanes.
Although insufficient stability in storage is a general problem of polyisocyanates and therefore well known in principle to the expert, in the case of solid pulverulent isocyanatosulphonic acids, the impairment of quality occurs after such a short time, e.g. in some cases after only a few days, that it makes the preparation of polyaddition products which can be used for practical purposes very difficult if not impossible.
On the other hand, there is a technical and commercial demand for the use of polyisocyanatosulphonic acids in polyaddition chemistry in addition to or instead of the usual diisocyanates and polyisocyanates because they are excellent starting materials for the production of hydrophilic polyurethanes, in particular polyurethanes which can be dispersed in water. Use of polyisocyanatosulphonic acids also appear to be particularly advantageous from the point of view of industrial hygiene or for physiological reasons because they have no vapor pressure and, on degradation, yield water-soluble amino sulphonic acids. The production of polyurethanes from sulphonated tolylene diisocyanate has hitherto been achieved either by sulphonating the prepolymer instead of the pure diisocyanate from which it is obtained as described in U.S. Pat. No. 3,826,769 or by preparing the isocyanatosulphonic acid only shortly before it was further processed to the polyurethane as described in U.S. Pat. No. 3,826,769. The first method has the disadvantage that it is restricted in the use of sulphonating agent, e.g. sulphur trioxide produces signs of decomposition in its action on polyether prepolymers. If sulphuric acid is used as sulphonating agent, chain lengthening with the formation of urea groups inevitably takes place at the same time. Furthermore, this method can only be used for sulphonating completely or partially free isocyanates but not products in the form of urethanes. This means that in an isocyanate prepolymer, only the isocyanate units present as end groups are sulphonated. The second method is not practicable on a commercial scale, because the manufacturer of a polyurethane cannot be expected first to carry out an isocyanate sulphonation.
It has also been proposed in U.S. Pat. No. 3,826,769 to dissolve the isocyanatosulphonic acids in an organic solvent, e.g. acetone, immediately after their preparation and then use them as solutions. This method also cannot be carried out on a commercial scale because a solution, for example of sulphonated tolylene diisocyanate in acetone, is not stable for more than a few hours at the most, after which the solution rapidly becomes cloudy and forms a precipitate.
It is, therefore, not surprising that solid isocyanatosulphonic acids have not so far become widely used in practice.
The problem, therefore, existed of preparing isocyanatosulphonic acids or stabilizing them so that they could be stored and used problem-free and their solubility in organic media would be maintained even after prolonged storage.
The present invention points a way to solving this problem. It has surprisingly been found that isocyanatosulphonic acids can be effectively stabilized by the addition of a substantially apolar, solid substance which is inert towards isocyanate groups and sulphuric acid groups and which has a melting point in the region between about 27.degree. and 250.degree. C. and which in the molten state is preferably a non-solvent for the aromatic isocyanatosulphonic acids, or, in other words, acts as a dispersing agent. The addition of only about 5% by weight of the solid dispersing agent is sufficient for stabilization, and the powder obtained does not fly up as dust.